1. Field of the Invention
Embodiments of the invention are generally directed to the field of image processing, and more particularly to methods and apparatus for the simultaneous measurement, analysis, and display of ocular wavefront information referred to herein as “online” aberrometry.
2. Description of Related Art
There are few technologies that have not been touched by the science of image processing. Advancing development in most fields typically gives rise to process automation where the ability to quickly and accurately detect structures in camera images becomes increasingly important. One area targeted by the embodiments of the instant invention is wavefront aberration measurements of the human eye, although the techniques set forth herein below will find much wider application directed to the detection of structure in images.
The field of ophthalmology concerned with vision correction through refractive surgery, or the use of lens components on or in the eye, has experienced the relatively recent development of aberrometers. These devices allow practitioners to explore the meaning of vision quality and to appreciate that it is not limited merely to visual acuity. Aberrometers which incorporate wavefront sensors can provide information about vision defects that, upon correction, may not only provide visual acuity at the theoretical limit but also better vision, perhaps even customized vision, under a panoply of viewing conditions.
One of the most conventional and well developed forms of ocular wavefront sensing relies on the Hartmann-Shack principle. A Hartmann-Shack wavefront sensor typically includes a microlens array that images various portions of a distorted wavefront exiting the eye onto a CCD detector/camera. The image produced by the microlens array comprises an array of small dots of light that are slightly displaced from reference locations of the light dot image from an unaberrated wavefront. The aberrated dot displacements are related to the localized slopes of the wavefront exiting the eye's pupil. Zernike polynomials (or other mathematical forms) can be derived from these displacements, which are then used to characterize virtually all of the eye's aberrations. The ability to make accurate wavefront calculations is critically dependent upon the true determination of the center location of each dot in the wavefront image. This aspect of the wavefront analysis process is known as centroid detection.
Hartmann-Shack wavefront sensors, and other well-known types such as Tscherning, for example, typically measure single images of centroids or, at best, a very small number of images over a short time interval. The eye, however, is a dynamic system with rapidly varying wavefront changes. The time needed for centroid detection has been the primary culprit hindering real-time measurements with repetition rates greater than a few images per second. A system known in the wavefront art as WASCA has demonstrated a repetition rate of about 7 Hz for the 30-second record of a wavefront. However, the wavefront images must first be recorded, saved, and subsequently evaluated. A single wavefront image requires about 400 Kb of computer memory. Moreover, aberration measurements (e.g., sphere, cylinder/axis, and higher order aberrations) cannot be displayed online, i.e., substantially simultaneously with the wavefront measurement and calculation. Nor is it possible to acquire and save pupil images and centroid images substantially simultaneously, making it virtually impossible to evaluate the influence of eye movement on changes in the wavefront. These illustrations represent some of the exemplary development issues in the field of ocular wavefront measurement addressed by the embodiments of the instant invention.